The term "promoter" as used herein refers to a signal on DNA for initiating the synthesis of mRNA with the DNA as a template, and it has a common sequence of characteristic nucleotides. In particular, DNA of eucaryote organisms has a common sequence designated "TATA box" about 20 nucleotides upstream from the initiation site for transcription, and this site is believed to be a necessary site for the initiation of transcription. For the purpose of producing a desired protein in large quantities, the use of a stronger promoter is believed to be advantageous. In general, 35S promoter of cauliflower mosaic virus (CaMV) has been frequently utilized in plants because of its high activity. This promoter has been actually used for producing herbicide-resistant plants or virus-resistant plants. However, 35S promoter has little tissue specificity, so that it may be requested to have tissue specificity depending upon the purpose of use. As a promoter having plant tissue specificity, many studies have been made on cis and trans factors. The use of a promoter having plant tissue specificity makes it possible to produce a transformed plant capable of controlling the expression of an introduced gene in a desired organ. At the present time, cotton fibers are produced by cultivating a cotton plant of the genus Gossypium and collecting the cotton fibers from the capsules (cotton balls) formed on the cotton plant. The cotton fibers are characterized by various physical properties among which fiber length, fineness, and strength are particularly important. Many efforts have been made in the past to improve the characteristics of cotton fibers. In these days, the development of gene engineering has given a possibility that the fiber characteristics can be altered by the transformation of cotton plants. In this case, it is very important to express a desired gene at a desired time in a desired tissue. Under the existing circumstances, however, the mechanisms of cotton fiber formation and elongation have not yet been fully elucidated, and the related genes or promoters have not yet been clearly revealed. In order to express a desired gene in a desired tissue or at a desired time, it is desirable to use a promoter with higher tissue or time specificity rather than a promoter always involved in the expression, such as CaMV 35S promoter. In particular, such a promoter is requisite for the improvement of cotton fibers.
Cotton fibers are elongated epidermal cells of ovules, and one fiber is made of one cell. The fibers are formed by the steps of initiation, elongation, secondary wall deposition, and maturation. In the past, several cotton plant-derived promoters have been found and reported to be useful for the improvement of fiber characteristics (see International Patent Publication No. WO 94/12014). For example, E6 promoter and B8 promoter were disclosed. In particular, detailed studies were made on the E6 structure gene, showing that E6 mRNA is strongly expressed in the fibers on the 15th day or later after the flowering. Northern blotting was long exposed to the mRNAs of various tissues with a probe derived from E6 cDNA, and weak signals were obtained in flowers, ovules, and leaves (Proc. Natl. Acad. Sci. USA 89, 5769-5773, 1992; in particular, see FIG. 1 of this reference). Furthermore, FbL2A promoter has been shown to be strongly expressed in the cotton fibers on the 25th day to the 30th day after flowering (Plant Physiol. (1996) 112:1331-1341). The strength and functioning time of promoters have been known to vary with the kinds of promoters. In the formation of cotton fibers, however, the 15th day or later after flowering corresponds to the later stage of fiber elongation; therefore, the improvement of fiber characteristics is not fully achieved only by these promoters, and it requires another promoter functionable within a period of from just after flowering to the 15th day after flowering. Even if it is a promoter functionable in any other tissue than cotton fibers, i.e., promoter with lower tissue specificity, it may be useful for this purpose.